pixman: update internal copy to pixman-0.28.2

Some w64 fixes by Stefan Weil found their way into 0.28.2,
so update the internal copy to that version to improve
windows support.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

pixman @ a5e5179b

-Subproject commit 97336fad32acf802003855cd8bd6477fa49a12e3
+Subproject commit a5e5179b5624c99c812e9bf6e7b907e355a811e8